Cancer is the second leading cause of death in the United States: 1 in every 4 deaths is attributable to the disease. Genetic and molecular profiling of tumor samples has lead to the identification of proteins that may be targeted by small molecules for therapeutic benefit. One such example is the p53 tumor suppressor pathway. The p53 gene remains wild type, yet the protein is functionally inactivated in approximately 50% of all human cancers. In these cases, overexpression of mdm2 and mdmx, the major negative regulators of p53, is frequently observed. Mdm2 is an E3 ubiquitin ligase, and catalyzes the transfer of ubiquitin from E2 ubiquitin conjugating enzyme(s) to p53. This in turn leads to proteasome-dependent degradation of p53. Many drug discovery efforts have focused on disruption of the mdm2/p53 interaction in order to increase p53 activity in tumors. However, current mdm2-specific antagonists cannot disrupt the interaction of mdmx with p53. Furthermore, recent data indicate that the interaction of mdm2 with mdmx, via so-called RING domains, engenders the most effective E3 ligase activity. Therefore, the identification of small molecules that can inhibit the mdm2/mdmx interaction may provide a new approach to p53 stabilization and activation in cancer. The Specific Aims of this proposal describe a cell-based screen of the MLCPN small molecule library to discover small molecule inhibitors of the mdm2/mdmx RING-RING interaction. Additional follow-up studies are presented that will validate the specificity of 'hits', and their efficacy in tumor cells that express wild type p53. The successful completion of these studies would provide proof-of-principle that small molecules can inhibit the mdm2/mdmx interaction, leading to stabilization and activation of p53. The chemical modification of primary hits in this screen will generate valuable chemical probes for basic scientific research, as well as potential lead compounds that may be taken forward for drug development. PUBLIC HEALTH RELEVANCE: Cancer ranks second to cardiovascular diseases in causes of death in the United States, and is a leading killer worldwide. The p53 tumor suppressor is wild type but functionally inactivated in ~50% of all cancers, and is an attractive target for reactivation by small molecule anticancer drugs. The Aims of this proposal describe an approach to identify a novel class of p53 activating compounds, and their successful completion will have implications for future treatment of cancers harboring wild type p53.